Question

What is a lunar month?
A) Month having 29 days
B) A lunar month is a time between two syzygies (new moon or full moon)
C) Moon’s orbital period in a non-rotating frame of reference
D) Average time for the moon to pass twice through the same equinox point of the sky.

Answer 1

Hint: A lunar calendar is a calendar based on the monthly cycle of the moon. The most used calendar is a solar calendar system which evolved from the lunar calendar. Like in the solar calendar system there are months in the lunar calendar. Every lunar month has 29.53 days.

Complete answer:
Consider option A. A lunar month does not have 29 days. And a month that has 29 days is related to the solar calendar system.
It cannot be a lunar month.
Consider option B. Here first let us see what syzygy is. Syzygy is a roughly straight-line configuration of three or more celestial bodies in a gravitational system. When it is a new moon it means the sun, earth, and moon are in a straight line and time between two new moons or syzygies is a complete cycle of the moon. This complete cycle is a lunar month of the lunar calendar.
Therefore the option B is correct.
In option C, what is a no-rotating frame of reference? First, let us understand what a rotating frame of reference is. A rotating frame of reference is a special case of a non-inertial reference frame that is rotating relative to an inertial reference frame. An everyday example of a rotating reference frame is the surface of the Earth. Now if you consider a non-rotating frame of reference the orbital period of the moon will not be equal to the time taken by the moon to complete a month cycle.
Therefore this option is also incorrect.
There are only two equinoxes when the sun rises exactly from the east and sets exactly in the west. When it is equinox the day and night are equal. The earth itself takes a year to reach the same equinox. Therefore the moon will take more time than a month.
Therefore the option D is also incorrect.

Note: The moon takes equal time to complete its rotation and to complete the circle around the earth therefore, the moon always shows the same face to the Earth. That's why you never see its far side. A new moon occurs every 29.5 days on the other hand solar calendars, such as the Gregorian calendar, track time using tropical years. A tropical year, also called a solar year, measures the length of time between two vernal equinoxes. That time period is 365 days.